Introduction

The foundations of item response theory (IRT) and classical test theory (CTT) were laid in two journal articles published only one year apart. In 1904, Charles Spearman published his article “The proof and measurement of two things,” in which he introduced the idea of the decomposition of an observed test score into a true score and a random error generally adopted as the basic assumption of what soon became known as the CTT model for a fixed test taker. The role of Alfred Binet as the founding father of IRT is less obvious. One reason may be the relative inaccessibility of his 1905 article “Méthodes nouvelles pour le diagnosis du niveau intellectuel des anormaux” published with his coworker Théodore Simon inl’Année Psychologie. But a more decisive factor might be the overshadowing of his pioneering statistical work by something attributed to him as more important in the later psychological literature—the launch of the first standardized psychological test. Spearman’s work soon stimulated others to elaborate on his ideas, producing such contributions as the definition of a standard error of measurement, methods to estimate test reliability, and the impact of the test lengthening on reliability. But, with the important exception of Thurstone’s original work in the 1920s, Binet’s contributions hardly had any immediate follow-up.